The invention relates to a piston of a rotary piston engine of trochoid type, having an internal seal with a scraper ring of L-shaped cross section arranged axially movable in a concentric annular groove in one face of the piston and held by spring action against the adjacent end wall of the housing, the scraper ring being sealed inside the annular groove by an elastic sealing ring arranged between the axial flange of the scraper ring and at least one wall of the groove.
In pistons of the kind described, the function of such internal seals is to seal off the piston against the end wall of the housing so as to prevent passage of coolant or lubricant radially outward not only along the end wall of the housing but also through the annular groove. For this purpose, in known internal seals the scraper ring is sealed off inside the annular groove using a sealing ring of elastomer material arranged between the axial flange of the scraper ring and one wall of the groove. This solution, however, leads to an unsatisfactory result because the elastomer material of the sealing ring becomes hard and stiff in prolonged service because of insufficient heat endurance, thus losing its geometrical stability and elasticity. As a result, the necessary mobility of the scraper ring is inhibited, thus limiting and failing to ensure the sealing function of the internal seal in the long run.
To avoid a seal depending on rubber elasticity of this kind, it is known that a scraper ring may be sealed off radially against one wall of the groove by means of a ring of metal spring material, C-shaped in cross section. In this version, however, it may happen that the radial oscillations of the scraper ring, due to the motions of the piston, are transmitted to the C-shaped ring and possibly curtail its service life, since the requisite elasticity of the C-shaped ring is limited by the small structural depth available in the annular groove.